This invention relates to a scraping blade used in a thin film dryer adapted for processing radioactive waste exhausted from a nuclear power plant or the like.
A thin-film scraping type dryer for chemical use has been widely used for drying radioactive waste exhausted from a nuclear power plant or the like, because the dryer can be easily constructed into a gas-tight device, and maintenance and inspection thereof are also easy.
The thin-film type dryer generally designated at 10 in FIG. 1 is ordinarily constructed as of the vertical type having a rotor 2 rotatable in the dryer 10 when driven by an electric motor 1 provided at the upper portion of the dryer. The cylindrical portion 13 of the dryer 10 includes an inner wall 4 and an outer wall 5 concentric with each other. The space between the two walls 4 and 5 is used as a heating-steam circulating jacket 6 having an inlet port 7 and an outlet port 8 provided at the upper and lower portions of the jacket 6. A number of blades 3 of a special design are arranged around the rotor 2 in an equally spaced apart relation, but in a closely adjacent relation when viewed from a side of the rotor 2. Furthermore, the blades 3 are secured around the rotor 2 in a swingable manner so that the blades 3 are directed radially outwardly by a centrifugal force when the rotor 2 is rotated around its vertical axis.
A radioactive waste exhausted from a nuclear power plant or the like in the form of an aqueous solution is introduced through an inlet port 9 provided at the upper portion of the dryer 10 into a distributor 12. Then the waste solution flows down along the inner wall 4 in the cylindrical portion 13 of the dryer 10 to be subjected to heating, evaporating, and drying processes, successively. More specifically, when the solution passes through a narrow gap between each blade 3 and the inner wall 4, the blade 3 directed radially outwardly by the centrifugal force extends the waste solution over the internal surface of the heat conductive inner wall 4 in the form of a thin film. The solution is thus dried out by the heat conducted through the inner wall 4 into a slurry, and then into solids which are dropped downwardly through a skirt portion 15 into a hopper portion 16 of the dryer 10. The water component of the aqueous solution is discharged through another outlet port 11 also provided at the upper end of the dryer 10 to the outside.
The waste discharged from an atomic power plant or the like contains various chemical compounds, of which hematite .alpha.-Fe.sub.2 O.sub.3 shows a Vickers hardness of approximately 500 Hv. To be operated with this compound and also with boron having a comparatively higher hardness and a large grain size, the scraping blades heretofore used are made of stainless steel having a Vickers hardness of approximately 150 Hv. Since the operative edges of the blades 3 are scratched at a high speed of from 8 to 12 m/sec. by the afore-mentioned components and elements contained in the waste while it is dried out on the inner wall 4, the blades 3 are abraded and worn out in a short period, thus requiring replacement after only one hundred hours of usage, for instance.
However, since the interior of the dryer is heavily contaminated by the radioactive waste, the replacement of the blades is extremely difficult. Furthermore, the replacement requires a considerably long period, and therefore the working time of the dryer in cooperation with the plant is much restricted.
Detailed construction of a conventional blade 3 is shown in FIGS. 2, 3 and 4.
The blade 3 comprises a main portion 17, a sharpened edge portion 18 formed outwardly of the main portion 17, and hinges 20 provided inwardly of the main portion 17 as viewed in FIG. 1. Each of the hinges 20 has a hole 19 extending vertically to receive a hinge pin (not shown), so that the blade is swingable horizontally around the hinge pin.
The edge portion 18 of the conventional blade is coated with a nickel-chromium self-fluxing alloy (Colmonoy) by a molten metal spray technique to increase the hardness of the edge portion 18. However, the Vickers hardness of the Colmonoy is approximately 600 Hv. which is comparable with the hardness of a waste solid obtained in the dryer 10. As a result, the operational life of this type of blade could not be much improved over the conventional blades made of stainless steel and having an operational life of approximately one hundred hours.
Moreover, the blade 3 shown in FIG. 3 is relatively weak against the shearing force caused by a centrifugal force in a time when the edge portion 18 of the blade scrapes a thin film of a dryer.
FIG. 4 shows an example in which the main body 17 of the blade and the chip 21 welded to an outer edge portion 18 of the main body 17 by brazing are made of different materials having different degrees of hardness. Inwardly of the main body 17 as viewed in FIG. 1, there are provided hinges 20 having holes for receiving hinge pins (not shown). With this example, as the blade is subjected to a considerably large shearing force caused by a centrifugal force, the blade of the type shown in FIG. 4 cannot sufficiently absorb such shearing stress and the chip merely welded to the main body 17 in likely to be cracked.